U.S. Pat. No. 3,593,188 discloses techniques for amplitude and frequency modulating a CO.sub.2 laser, but does not disclose the combination of amplitude modulation (hereinafter "AM") and frequency modulation (hereinafter "FM") of a single laser beam as in the present invention.
U.S. Pat. No. 3,940,712 shows AM and FM of a laser. However, the present invention differs from this patent in that (1) AM and FM of a laser can be simultaneously present; (2) electro-optic (hereinafter "EO") crystals are used, rather than slow mechanical modulators; (3)the amplitude modulator is preferably disposed within the laser cavity, providing for a more efficient system than for the outboard amplitude modulator of the patent.
U.S. Pat. No. 3,569,856 shows FM of a laser, but not AM.
U.S. Pat. Nos. 3,918,007; 3,229,223; 3,487,230; and 3,622,912 disclose techniques for AM of a laser, but not FM.
With respect to the systemic aspects of the present invention, a brief tutorial on sinusoidal AM and FM radar is appropriate. In sinusoidal AM radar, the measurement of range between the transmitter and the target depends upon a determination of the phase difference between the sinusoidal AM signals received and transmitted. The accuracy of this measurement of the phase difference depends upon the signal-to-noise ratio in the radar receiver, which is in turn a function of transmitter power, output beam aperture, and pixel scan rate. In general, absolute range is uncertain with a sinusoidal AM radar. The absolute range resolution is subject to a range ambiguity interval (or "ambiguity range" r.sub.a), which is determined by the frequency of the sinusoidal AM. For a sinusoidal AM radar, the ambiguity range r.sub.a =c/2f.sub.m where c is the speed of light and f.sub.m is the modulating frequency. As f.sub.m gets higher, the absolute resolution gets better (smaller), but the ambiguity range undesirably gets lower as well.
In an FM radar, on the other hand, the measurement of range depends upon a determination of the frequency difference between the signal received and the signal transmitted. Because of the phase decorrelation of the received signal as a result of scanning over the scattering target area (which problem is particularly acute for laser radars, because they must scan at high pixel rates relative to microwave radar to cover the same angular field of view), the spectral width of the received signal is broadened by approximately the reciprocal of the pixel dwell time. For a typical laser radar, the amount of broadening is between 50 Khz and 500 Khz. This limits the accuracy and resolution that can be achieved in the receiver. If one considers the FM rates possible in a laser transmitter, this corresponds to a range accuracy of between 30 meters and 200 meters. Thus, an FM radar typically has worse range resolution than an AM radar. However, it does not have the ambiguity difficulties and the backscatter problems inherent in the AM radar.
The primary prior art approach for removing ambiguities in a microwave sinusoidal AM radar system is to send discrete pulses of sinusoidal AM at stepped frequencies. By contrast, the present invention is capable of simultaneously modulating a radar beam with continuous AM and continuous FM. This offers the following advantages over the prior art: (1) Absolute range is measured, thus intervening atmospheric backscatter can be discriminated against. (2) Ranging and imaging of moving targets is facilitated, since the change in the frequency excursion, .DELTA.f, of an individual sample is twice the Doppler frequency shift attributable to target motion. (3) The prior art approach would be difficult to implement in a laser: step or discrete changes in the frequency of a laser are difficult because these require very fast optical or mechanical changes in the laser cavity optical path length. (4) The power required to activate the amplitude and frequency modulators (21, 25) in this invention is less than for other prior art techniques including pulse compression methods using very high FM chirps.
In a second prior art technique for eliminating the range ambiguities associated with AM radar, the AM is transmitted at several frequencies simultaneously. This technique is popular for visible lasers, but would use vastly more power than the present invention.
These prior art systems are discussed in Skolnik, M. I., Introduction to Radar Systems (2d ed. 1980), McGraw-Hill Book Company, pp. 95-100.
In summary, the prior art does not suggest the combination of continuous amplitude and continuous frequency modulation of a single radar beam.